How many turrets did a Fletcher class destroyer have?
More than a modern destroyer, no matter who makes it - but a modern destroyer has a much greater throw weight and range due to its missile batteries.
Fletchers had 5x 5 inch/38 calibers cannons in single turrets. Most modern destroyers carry evolved 5” or similar guns a single turret mounting, sometimes double. Their main firepower is in their missiles.
Oh, yeah, forgot to mention - our latest US destroyer is the size of a pre-dreadnaught battleship in most dimensions and only has one main gun - and that can’t direct fire on targets. The gun doesn’t work because we didn’t buy any ammo for it.
A modern destroyer (other than the DDX-1000 or those stupid LCS that they’re pretending are destroyers) could wipe out a Fletcher from beyond any range the Fletcher could respond at. (In fact, a modern destroyer could hose an entire WW2 Iowa-class and most of its battle group in a duel, which is part of why the Iowas were withdrawn for the last time.) The only way a Fletcher would win is if somehow the enemy destroyer let the Fletcher get to its gun range. And even then it’s questionable as modern CIWS systems are perfectly capable of shooting WW2 5” shells *out of the air*.